Eukaryotic cells duplicate their chromosomes during S-phase, and segregate the chromosomes into two daughter cells during mitosis. Errors in the chromosome segregation could lead to gain or loss of chromosome. This is a cause of chromosome instability which is a hallmark of tumorigenesis. Our long-term goal is to understand the molecular mechanism of accurate chromosome segregation in S. cerevisiae. Orc6p controls DNA replication from yeast to humans. Recently it has been proposed that Orc proteins might also have mitotic functions besides its well characterized function in DNA replication. Human Orc6p is localized to kinetochore, and has been implicated in the control of chromosome segregation. However it is not clear how Orc6p controls mitosis. I hypothesize that Orc6p has a mitotic function in S. cerevisiae. To study the mitotic function of Orc6p in S. cerevisiae;(1) a conditional mutant of orc6 will be isolated and characterized. (2) Protein localization will be analyzed to see if Orc6p is localized kinetochore during mitosis. (3) A suppressor gene screening using the orc6 conditional mutant will be conducted to reveal the specific genetic interactions between ORC6 and mitotic genes. Successful isolation of orc6 mutant which is defective only during mitosis will help us to study the mitotic function of Orc6p. Kinetochore localization of Orc6p will ensure that Orc6p has a role in chromosome segregation. Suppressor gene screening will help us to understand the molecular mechanism of Orc6 function. Orc6p is conserved from yeast to humans. The results obtained from this study can be applied to understand the molecular function of Orc6p in higher eukaryotes. The accurate chromosome segregation is a critical step during mitosis to avoid chromosome instability. It is important to study the mechanism of chromosome segregation in eukaryotes to understand the molecular mechanism of tumorigenesis.